You are allowed two stopovers in Europe. A stopover occurs if th ebreak between consecutive flights is 24 hours or more.

You cannot buy more stopovers in your country of origin.

If you fly CAI-LHR-NBO then you have used one of the four segments you are allowed in Europe (CAI-LHR). If you stay in London less than 24 hours then you have not used a stopover, but if more than 24 hours then you have used one.

Note that if you start like this then you have to exit Africa either on the CX flight from JNB to HKG, or from ACC or NBO to North America or (I think) Asia via a transit (<24 hours) in London.